I'm not a professional plumber but my friend teach me how to stop running toilet easily. I thought this tips will help you to fix almost 90% of running toilet problems.
'''1. Check the toilet fill valve & tube:'''
Then main reason behind running toilet is the toilet fill valve, so [https://www.chriply.com/best-toilet-fill-valve/|get a good toilet fill valve] to avoid fill valve problem.
'''2. Adjust the float:'''
If the float isn't in the water level, adjust it and set to the water level. Keep adjusting it until the water shuts off at the perfect level.
'''3. Adjust the chain & handle:'''
Sometimes loose or hard chain & handle cause running toilet, cut off the excess chain and adjust it properly.